Two sub-pixel rendering algorithms based on the three primary colours are proposed, which add much more green sub-pixel to the traditional RGB structure. They transfer the RGB data of original image into new RGBG data, and then rendering the converted RGBG data. The proposed algorithms ensure that the image has same resolution as the traditional RGB arrangement, and also increases the pixel per inch (PPI). Simulation results show that, the proposed two algorithms can keep the clarity and saturation of original image, and increases the PPI by 50%, with more exquisite and smooth display effect.